Standard sand blasting equipment consists of a pressure vessel or supply pot to hold particles of a blasting medium such as sand, a source of compressed air connected to the supply pot via a conveying hose and a means of metering the blasting medium from the supply pot, which operates at a pressure that is the same or slightly higher than the conveying hose pressure. The sand/compressed air mixture is transported to a nozzle where the sand particles are accelerated and directed toward a workpiece. Flow rates of the sand or other blast media are determined by the size of the equipment. Commercially available sand blasting apparatus typically employ media flow rates of 10-20 pounds per minute. About 0.5 to 1 pound of sand are used typically with about 1.0 pound of air, thus yielding a ratio of 0.5 to 1.0.
When it is required to remove coatings such as paint or to clean relatively soft surfaces such as aluminum, magnesium, plastic composites and the like, or to avoid surface alteration of even hard materials such as stainless steel, less aggressive abrasives, including inorganic salts such as sodium chloride and sodium bicarbonate, can be used in place of sand in conventional sand blasting equipment. The media flow rate used for the less aggressive abrasives is substantially less than that used for sand, and has been determined to be from about 0.5 to about 10.0 pounds per minute, using similar equipment. The lower flow rates require a much lower media to air ratio, in the range of about 0.05 to 0.5.
However, difficulties are encountered in maintaining continuous flow at these low flow rates when conventional sand blasting equipment is employed. The fine particles of an abrasive media such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, the bicarbonate media particles tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. Flow aids such as hydrophobic silica have been added to the bicarbonate in an effort to improve the flow, but maintaining a substantially uniform flow of bicarbonate material to the blast nozzle has been difficult to achieve. Non-uniform flow of the blast media leads to erratic performance, which in turn results in increased cleaning time and even to damage of somewhat delicate surfaces.
Commonly assigned U.S. Pat. Nos. 5,081,799 and 5,083,402 disclose a modification of conventional blasting apparatus by providing a separate source of line air to the supply pot through a pressure regulator to provide a greater pressure in the supply pot than is provided to the conveying hose. This differential pressure is maintained by an orifice having a predetermined area and situated between the supply pot and the conveying hose. The orifice provides an exit for the blast media and a relatively small quantity of air from the supply pot to the conveying hose, and ultimately to the nozzle and finally the workpiece. The differential air pressure, typically operating between 1.0 and 5.0 psi with an orifice having an appropriate area, yields acceptable media flow rates in a controlled manner. The entire contents of U.S. Pat. Nos. 5,081,799 and 5,083,402 are herein incorporated by reference.
A media metering and dispensing valve which meters and dispenses the abrasive from the supply pot through the orifice and to the conveying hose carrying the compressed air stream typically operates automatically whenever the compressed air is applied to the blast hose to begin the abrasive blasting operation. The media valve for use in the afore-mentioned metering and dispensing process as disclosed in U.S. Pat. Nos. 5,081,799 and 5,083,402 is characterized as a Thompson valve and is described generally in U.S. Pat. No. 3,476,440, the contents of which are herein incorporated by reference. The Thompson valve includes a metering valve stem which blocks the outlet of a discharge tube disposed between the supply pot and an air flow tube which is secured to and carries the compressed air to the conveying hose. When the blast nozzle is activated, the valve stem is lifted from the valve seat of the Thompson valve and allows a controlled amount of media to flow through the outlet of the discharge tube into the air flow tube. The valve as disclosed in U.S. Pat. No. 3,476,440 has been improved by placing a plurality of orifices having different sizes within a control sleeve which is placed around the end of the valve stem. One of the orifices can be placed in communication with the outlet of the discharge tube and the air flow tube. When the valve stem is placed fully within the control sleeve, the orifice in the control sleeve is blocked such that media cannot flow from the discharge tube through the orifice in the media control sleeve and then into the air flow tube. Upon operation of the blast nozzle, the valve stem is lifted within the control sleeve and pulled away from the orifice to allow the media flow from the pot to the discharge tube, through the orifice and into the air flow tube.
The plurality of orifices provides another means of controlling the amount of media flowing from the supply pot into the compressed air stream and into the blast nozzle apparatus. Unfortunately, to change the orifice which is in alignment with the media discharge tube and the air flow tube or to clean out a plugged orifice in the Thompson valves now on the commercial market, it is required that the valve body holding the sleeve be taken apart, the control sleeve taken out, rotated, placed back in its slot and the valve body then restructured. Obviously such disassembly and reassembly is cumbersome and certainly does not allow for efficient blast cleaning on the job site.
The present assignee has developed a novel and improved media control valve which is particularly useful in the differential pressure metering system of U.S. Pat. Nos. 5,081,799 and 5,083,402. The improved metering valve offers additional control with respect to metering the flow of media. Like the prior art Thompson media control valves, the novel valve includes a control sleeve which contains a plurality of orifices, one of which can be aligned to communicate with the discharge of the media from the supply pot and the air flow tube to dispense the media into the compressed air stream. The plurality of orifices have a different diameter to allow enhanced control of the amount of media dispensed from the supply pot to the compressed air flow tube by allowing a change of orifice size. Importantly, however, the control sleeve is made longer to encompass the valve stem and is attached to the valve body. To control the metering of the abrasive media into the air flow tube, the control sleeve can be rotated from the exterior of the valve body while still in place in the valve body to align a different orifice with the media discharge passage in communication with the supply pot and the compressed air flow tube. Alternative embodiments are provided to index the control sleeve such that an orifice is properly aligned upon rotation of the control sleeve. In one embodiment, the index means comprises a ball spring plunger placed in the valve body and exerted against the control sleeve and a series of detents spaced in the sleeve and aligned with each orifice so as to properly align the orifice with the media discharge passage from the supply pot and the air flow tube when the ball spring plunger fits within a detent in the sleeve. The control sleeve and the enclosed valve stem can be easily removed from the valve body in one piece for cleaning and replaced and locked in place in the valve body by means of a lock pin without disassembling the body of the valve. In the second embodiment, the index means comprises a plurality of grooves which are placed on the face of the bore which receives the control sleeve and which mate with a plurality of teeth on the control sleeve. The teeth are aligned with the orifices. To change orifices, the control sleeve is lifted to disengage the teeth from the grooves and rotated until the teeth and grooves are again aligned and the sleeve then dropped in place in the valve body. The media control valve also includes a manually adjustable multi-port valve placed within the media discharge passage and which can close off the discharge passage from the supply pot, and allow compressed air to back clean the valve and direct debris out a clean-out port in the valve body. The novel and improved media control valve is described in commonly assigned, copending application, U.S. patent application Ser. No. 161,530, filed Dec. 6, 1993, the entire contents of which are herein incorporated by reference.
Below the orifice in the improved media control valve disclosed in U.S. patent application Ser. No. 161,530, is a media discharge tube which directs and feeds the abrasive media from the orifice into the air flow tube. In accordance with an improvement in such valve as disclosed in copending, commonly assigned U.S. patent application Ser. No. 229,011 filed Apr. 18, 1994, the media discharge tube converges downstream of the orifice to the outlet thereof in the air flow tube such that outlet is shaped as a slot-like outlet of elliptical to rectangular shape. The length of the outlet slot into the air flow tube runs in the same direction as the compressed air passing through the air flow tube. Accordingly, the perimeter of the outlet placed perpendicular to the direction of compressed air passing through the air flow tube is minimized which consequently greatly reduces the air turbulence in the air flow tube at the outlet of the media discharge tube. Reduced turbulence at the media outlet into the air flow tube reduces variations in the air line pressure in the media discharge tube immediately below the orifice providing for accurate and consistent differential pressure across the orifice and consequent uniform metering of the abrasive media into the compressed air stream.
Importantly, the media outlet of the media control valve disclosed in U.S. patent application Ser. No. 229,011 unlike previous media control valves which dispensed the abrasive media into the center regions of the air flow tube wherein the compressed air speed was the fastest and the air, the most turbulent, dispenses the abrasive media tangential to the circumferential inner surface of the air flow tube where the air is most likely in laminar flow due to boundary effects along the inner tube wall. Injection of the media into the low speed air along the inner wall of the air flow tube minimizes turbulence of the abrasive media so as to maintain the integrity of the individual abrasive particles and additionally reduces valve wear at the outlet to the air flow tube.
While the improved media valves of above-mentioned U.S. patent application Ser. Nos. 161,530 and 229,011 are convenient to use and can very accurately dispense an abrasive media such as sodium bicarbonate into an air stream, there has occurred several problems when using the valves over extended periods of time. It has been found that abrasive particles and dust can accumulate between the media control sleeve and the valve stem which slides therein to open and close the orifices at the end of the media control sleeve. Accordingly, over time, this abrasive has been found to cause wear on the valve stem which results in the leakage of more abrasive particles into the space between the media control sleeve and the valve stem and further has prevented the operation of the valve stem as too much wear and too much accumulated abrasive dust often can hinder movement of the valve stem.
Upon the addition of the control sleeve, the valve seat was eliminated. Accordingly, very tight tolerance between the valve stem and the interior of the control sleeve has been necessary to prevent unwanted media flow. Abrasive wear on the valve stem disrupts the tight clearance between these valve components. Moreover, if the media control sleeve is not accurately machined and aligned with the bore which receives the valve stem in the pneumatic chamber which activates the valve stem, this again can result in substantial wear on the valve stem as it moves through the media control sleeve. Even a minor misalignment can disrupt the tight clearance and prevent the smooth operation and movement of the valve stem through the control sleeve resulting in excessive wear, leakage of abrasive dust in the bore of the media control sleeve and actual stoppage of the valve stem from passing totally through the media control sleeve.